Yabghu (Old Turkic: ?, yab?u,Traditional Chinese?, Simplified Chinese?, Jabgu, Djabgu, literally, "pioneer", "guide") or Yabgu was a state office in the early Turkic states, roughly equivalent to viceroy. The title carried autonomy in different degrees, and its links with the central authority of Khagan varied from economical and political subordination to superficial political deference. The title had also been borne by Turkic princes in the upper Oxus region in post-Hephthalite times.
The position of Yabgu was traditionally given to the second highest member of a ruling clan (Ashina), with the first member being the Kagan himself. Frequently, Yabgu was a younger brother of the ruling Kagan, or a representative of the next generation, called Shad (blood prince). Mahmud Kashgari defined the title Yabgu as "position two steps below Kagan", listing heir apparent Shad a step above Yabgu.
As the Khaganate decentralized, the Yabgu gained more autonomous power within the suzerainty, and historical records name a number of independent states with "Yabgu" being the title of the supreme ruler. One prominent example was the Oguz Yabgu state in Middle Asia, which was formed after the fragmentation of the Second Türkic Kaganate in the 840es. Another prominent example was the Karluk Yabgu, the head of the Karluk confederation which in the 766 occupied Suyab in the Jeti-su area, and eventually grew into a powerful Karakhanid state.
There are at least five theories among recent literature regarding the origin of yabgu.
Others, such as Sims-Williams, considered that the word yabgu in Turkic languages had been borrowed from Old Chineseip-g'u > x?hóu, rendered in Chinese characters as  or  Conversely, Friedrich Hirth suggested that yabgu was transcribed literary Chinese, with regard to Kushan and Turkic contexts, as *xiap-g'u > x?hóu. It was equivalent to the title yavugo found on Kushan coins from Kabul, and the yabgu on ancient Turkic monuments. The second part of this compound Chinese word, hou ("g'u"), referred to the second-ranking of five hereditary noble ranks. Chinese sources do not make clear whether the title was a descriptive term used only in reference to foreign leaders, or whether it indicated an ally or subject of a Chinese empire.
Another theory postulalates a Sogdian origin for both titles, "Yabgu" and "Shad". The rulers of some Sogdian principalities are known to have title "Ikhshid".
^Hou Hanshu "Vol. 88: Greater Yuezhi nation" text "?,?,,," translation "Formerly, the Yuezhi were defeated by the Xiongnu. They then moved to Daxia (Bactria) and divided up this kingdom between five xihou ('Allied Princes'), which were those of Xiumi (Western Wakh?n and Zibak), Shuangmi (Shughn?n), Guishuang (Badakhsh?n and the adjoining territories north of the Oxus), Xidun (the region of Balkh), and Dumi (the region of Termez)." by John E. Hill.